This invention pertains generally to a method for improving the accuracy of particle analysis under conditions of discrete particle loading and particularly to a method for improving signal-to-noise ratio and instrument response in laser spark spectroscopic analysis of particulate emissions.
Thermal processing, including incinerator and plasma processes, is a viable method for the treatment of a wide variety of materials including waste materials. Many feeds, however, contain quantities of metal compounds that cannot be destroyed during the primary treatment process, but instead must be removed from the effluent stream by conventional scrubbing techniques. Significantly, the Clean Air Act regulates eleven metals that have been identified as air toxins: antimony (Sb), arsenic (As), beryllium (Be), cadmium (Cd), chromium (Cr), cobalt (Co), lead (Pb), mercury (Hg), manganese (Mn), nickel (Ni), and selenium (Se). In view of the utility of thermal processing and the need for regulatory compliance, a technology for continuous monitoring of metal emissions would be desirable.
A candidate for such a continuous particle monitor can be based on a technique referred to as Laser Spark Spectroscopy (LASS) or Laser Induced Breakdown Spectroscopy (LIBS). French et al. in co-pending application (Ser. No. 08/228,974), incorporated herein by reference, disclose a LASS-based method and apparatus for analyzing particulate emissions from combustion systems. In particular, this system comprises a means for rapidly heating a particle or assemblage of particles to form a plasma, a means for collecting and transmitting light emitted by the plasma, a means for optically dispersing the light into wavelength components, a means for measuring the distribution of light intensity as a function of wavelength to produce spectral data, and a means for acquiring, analyzing, manipulating and displaying the spectral data. As disclosed by French, et al., this technique employs, a pulsed laser beam is focused in the effluent stream where the high energy and power densities of the beam generate an optical breakdown, also referred to as a laser spark or laser-induced plasma. If the laser beam is focused onto a particle, all species comprising the particle, including the metals of interest, are decomposed into excited atoms and ions. The spectral emission from these excited atoms are captured by standard optical elements and directed into a spectrometer where the emitted light is dispersed into wavelength components. Measurement of these components enables species identification, while the emission intensities provide a measure of the atomic concentrations.
The LASS system response to a specific metal can be calibrated to reflect the metal concentration, usually expressed in the units of .mu.g/acm (acm=actual cubic meter). In reality, the response corresponds to the actual mass of metal contained within the actual plasma volume. The plasma volume is typically 5.times.10.sup.-5 cm.sup.3. Instrument calibration can be accomplished using fine aerosol dispersions of known concentrations for each metal of interest. When the metals are dispersed on a length scale that is much smaller than the minimum plasma dimension (250 .mu.m), each laser spark contains a representative metal sample, and multiple-shot averaging may be used to reduce single-shot experimental noise. Using such an approach, the LASS instrument can be calibrated and the instrument response, including minimum detectable concentrations, can be determined for various metals.
In combustion systems, however, the fate of metals is a complex phenomenon controlled by mechanisms such as particle entrainment, chemical interactions, vaporization, condensation, particle coagulation, and particle collection. Furthermore, in the effluent stack, where LASS monitoring would typically take place, most metals exist as either homogeneous or multi-species particulate. Under such discrete particle loading conditions, uniform metal sampling via LASS sampling is by no means guaranteed. In fact, the conditions of low particle density loading resulting from low overall metal concentrations and/or large particle sizes can be a limiting factor for the success of a LASS metal emissions monitor. What is needed, therefore, is a technique to deal specifically with the issues of variable particle loading and resulting poor instrument response.